Process for obtaining by galvanic means a coating for protecting magnesium from corrosion



Patented May 15, 1951 U NIT ED STAT PATENT LC E. 1

PROCESS FOR OBTAINING. BY" GALVANIC MEANS A. COATING FOR PROTECTING"MAGNESIUM FROM CORROSION HerbertManfred Freud, Nanterre, France NoDrawing. Application April'18,'1946, 'Se'r'ial' No. .663, 24'8. InFranc'eJuIy-ZB, 1945 6 Claims. (Cl. .204--56 1, In my copendingapplication (on which Patent No. 2,463,483 issued March 1,1949) SerialNo.

314,349, filed January 17,1940, I described a procr.

ess for obtaining by. galvanic. means a coating.

for protecting magnesium from corrosion, such coating being produced bya galvanic process.

The preferred solution, which was used in that process contained the.bi-valent ion Mn++ associated with an anion which does. not attack themetal chemically, such as 01720:".

In the process. set forth in Ser..,.No.,3,l4,3,4.9, several manganesesalts suchIas the nitrates, the chlorides, the sulphates, were statednot to be. capable of producing a good protective layer on magnesium,because they produce an attack on the metal.

The improved process which is the object of the present invention isrelated to this same technique of action by galvanic means as opposedto. action by chemical means, and herein, as in Ser. 314,349, noelectric current from outside is supplied.

One of the greatadvantages of protecting magnesium by galvanic meansunder the conditions of the present case (in addition to the improvedresistance to corrosion) is that it is possible, in certainconditions,and in .particular by. the process which is the object of applicationSerial ,No.. 314,349, to provide an excellent. protective coatingwithout producing aconsiderable change of dimensions, when treatingcalibrated parts.

Before starting the description of the process which is the object ofthe present invention and in order to make more apparent the importanceof; this fundamental distinction which exists between the action bychemical means and action by galvanic means, some indications will firstof all be given concerning the phenomena which characterise the actionby chemical means in the reactions relating to the treatmentofmagnesium.

The chemical attack of magnesium is generally manifested by one of the.two,fo'1lowingphenomena:v

1.. A progressive dissolution of a surface-portionof the metal, withoutleaving. any chemically produced. covering occurs, and the amount of themetal so dissolved can be evaluated .by the loss in weight of the metalduring its immersion in the solution.

Thus, a plate of 1 *dm' (total surface :area, on

both sides) of an-alloy of magnesium contain-.

ing'1.8% of manganese was immersed in :a solutien containing 0l-6.-% rotiMnt ion .and;1-.0%=.oi; CIOs obtained zby dissolving .100:- :.gr.. of.CI'QB .in. 999120. of Water-tand .iadding..;thereto 19 gr; :of

MnCOs. Thisv soLutiomhasuaipI-I less .than 1.1 It is difficu-lt toaccurately determineizhegafl pf: this solution by the scientificmethodsiknownyat present, :owing. tolthe presence @iia large propor tionof 01103, thelex'idizingpowerioiwhich is;.:cone; siderable, butv the pH;isnlessi thazn .l,. measu-red any; the. glass e1ectro'de.l-.,Said zp-late weighed 9.125 gr; before it wasintroduced into thezbath, and8.944; gr. afterremainingxtherein enethour; The lossiofi. weighttherefore. .0384; -gr... The; decrease- "in; thicknessof the plate wasmore. than 2/100.

(i. .e. somewhat over 9.02 mun);

:2... Dissolution pf a qaortion .of the :.meta-l-- and; the.:simu1taneous...coaifing.; of; the metal with .wa layer of which:thexcelour varies from yellow to; mordo-r (brownish-.xmimson), brownor; black. This. layer. .ismore orzless; adherent to themagnesiumand-according tozitsthickness, is :of -great-.-. er;-;or;- lesser:weight-avhichqmay to. some extent; counterbalance the thickness and weiht of metal lostby solution.

Thuspa ,plate of 1 dm -itotalzsuraface :area, :on: both sides) ofansallny ,ccntaining.1:.8% or ganese was immersed for an hour in asolution containing 10% of 'Naz'croiand 8% of KHSO4 (pH=5.7). Said plate'initiallyiweighed 9.087 gr., and 9.039 gr. after-remaining in the bathfor one hour. During: that. treatment it becamev covered with a ,blackfilmdotted with-"yellow spots. Theapparent loss of weight was thereforeonly. Q 0 048 gr.

But, when this .blaek-filmwasremoved, by suc--- cessive immersions-in,hot: solutions oi soda and of. sodium hyposulphite :(NazSzOs. which isoommonly called hypoii, the-plate weighed -onl y- 8.954 gr. The rear.lossaof weight in the. metal i itself iis thereforeO .1 33 g-r eradecrease. of thick. ness of the metal-sheetnfzabout.0.0115 mm;

7 In the absence oiionsnther:thanMnH and; CrtOm-- or. ICIOPCT, thechemical attack takes: place only if the pH-of; the bath 'is :very low,i. .e. much-v -below 2:5. contradistinction, a the presence of-ions-suchars 0.4, theichemical attack takes placeat a pH ofrnearly'fiyorat any=pI-I lower than this if the bath aconsiderable;

concentration oflsuchzionsl.

My research 1 work has enabled me-ate. discover the-conditions under;which with maths-in which:

. the acid-ion is composed :principally .i0I.;.CIi2O11'0r;-:

CrO4 the galvanic action takes place without there being a chemicalattack on the magnesium, and on the other hand to discover that, as anexception to the general rule, mentioned in application Ser. No.314,349, and the accuracy of which has been practically confirmed bypractice, that the presence of sulphates, chlorides or nitrates producedan attack on the metal, the chemical action did not occur if the acidion concentration is maintained within certain limits hereinafterdefined.

Consequently, the object of the present invention is a process forprotecting, by galvanic action, metal objects, by means of a bath mainlycontaining as anion one of the CrzOv or CrO4 ions and which consists inoperating with certain conditions of pH and of concentration which aredefined hereinafter:

1. According to the invention, when the operation is effected in theabsence of ions other than Mn++ and CrzOq-- or CrO4 the pH should bemaintained, in order that the galvanic action may be effected insatisfactory conditions, between 2.5 and 3.5 for a fresh bath. It isadvisable in practice to keep the pH at about 3.

I have found that at a pH below 2.5 and at a pH much above 3.5, thetreatment is too slow for the application of the process to besatisfactory industrially.

2. The operation may be effected in the presence of very smallquantities of an active mineral acid, which will be obtained by theaddition of ions such as SO4--, N03", PO4-, either in the form of acids,or in the form of salts. In particular S04" gives good results whetherit is introduced in the form of sulphuric acid or of sulphates, or againin the form of sulphonated organic acids. Under these conditions (i. c.with a very small amount of H2804 added), an electrolyte having a pH upto 3.95 may be satisfactory for use, (see table below).

For this addition of these acid ions not to produce any chemical attack,the bath must be such that the potential at the magnesium electrodeformed by the object to be treated is less than 1.1 volts.

The concentration of such active mineral acid ions should be less than0.5% and in particular it should be less than the figures:

The pH should be higher than 2.5. The upper limit of 3.5 is mentioned asbeing inadvisable to exceed owing to the slowness of the galvanicaction, when the Mn++ and CIv or CrOrions alone are present in theelectrolyte. But this limit may be considerably exceeded in the presenceof sulphuric, nitric or phosphoric acids or their salts since thepresence of small quantities of the acid radicals of these acidsincreases the oxidizing power of the bath.

The table given hereinafter, which gives, for a series of proportions ofconstituents, the value of "the pH and that of the potential at themagnesium electrode, relatively to the hydrogen electrode, illustratesthe fact that according to the proportions, either a chemical attackoccurs (which is very undesirable), or, on the contrary, only a galvanicaction.

In practice, for these measurements, in view of the present difficultyin operating with hydrogen electrodes, I have used as a referenceelectrode a calomel electrode with saturated potasan electromotive forceof 1.9 volts.

Item Bath composition (per liter) E 10 grs. CrOa and 5.8 grs.

MnCO

3. Item a plus 0.1 cc. HzSO4.- Item a plus 0.3 cc. H2SO4 grs. CrOa and58 grs.

M11003. Item 11 plus 0.1 cc. HzSO4. l0 grs. CrOa and 6 grs.

nCO

Nam on NRO HUIN chemical attack.

The figures in the column marked E, in the above table show the computedpotential of the magnesium electrode relatively to the hydrogenelectrode.

If, instead of the S04 ion, N03 and P04 ions 'were used, a similar tablewould be obtained.

In the latter cases also chemical attack occurs when the potential atthe magnesium electrode relative to the hydrogen electrode, is greaterthan 1.1 volts.

It should be noted that the small quantities of N03 ions carried alongas a result of the usual depolarising operations of carbons by means ofnitric acid, followed by athorough washing, do not substantially alterthe limits given in either of the above mentioned cases. However, it ispossible, on the contrary, to limit the washing so as to allowsufiicient N03 ions to remain for the acidity to be maintained withinthe above mentioned limits.

By way of examples, given for the sole purpose of facilitating thecomprehension of the present description, it is possible to proceed asfollows:

Example I A solution is prepared by dissolving in 1 litre of water, 10gr. of CrOs, and 5.8 gr. of manganese carbonate are then added, and then0.100. of sulphuric acid. The pH of this bath is 3.2.

A plate of an area of 1 dm (total surface area, on both sides) ofmagnesium alloy containing the circuit, a current of 70 milliamps flows,at a potential of 0.9 volts. 7

After a suitable period of treatment, (e. g. one hour), this piece iswithdrawn, washed and dried; it is covered with a dark grey coating, andweighs 9.001 gr. The coating formed therefore weighs 0.026 gr.

Example II Into a solution prepared from 1 litre of water,

10 gr. of C103 and 6 gr. of MnCOs, there has been carried (from the.depolarisation of the.

By closing carbons Tinflhlilrlfhacldllflfil grlSofii HNOa The bath.,accordingly contains...about 0.05%...I-IhIQ3.

,The "pH of -this bath is 3.3.

. -.A plate of an area'of 1 dm lttotal surfacelarea,

mersion in the bath and 9.016 gr. after an immersion of "one-hour. Thereis therefore no chemical attack whatever.

.;.:-If carbon counterelectrode/is; placed opposite ithis magnesiumalloy plate, a-'cell iswobtained r having an electromotiveforce .(inopen'circuit) of 1.78 volts. By closing the circuit, a current of 55milliamps'fiows at 0.85 volt.

Aftera .suitable period Jof. treatment I hour, for example), the pieceis withdrawn; washed'and .nrlried ;It is covered. with albrownfilm:and:w.eighs The coating formed therefore-weighs Example "III To asolution prepared by dissolving 100 gr.

of chromic acid in 1 litre of Water, are added 66::

gr. of MnCOs, 1.2 gr. of MnSO4 corresponding to 0.75 gr. of S04 ion. ThepH of the bath is 3.7. A plate made of magnesium alloy containing 2.7%of aluminium, 0.3% of manganese and 1% of zinc and having an area of 1.2dm (total, for both sides of plate) is immersed therein.

The potential of the Mg relatively to the hydrogen electrode is 0.98volts. When the circuit is open the electromotive force between the 1 gand a counter-electrode of carbon is 1.88 volts. By closing the circuita current of 60 milliamps flows at 0.90 volts. After a period oftreatment of hour, the piece is withdrawn, washed and dried.

The durations of the galvanic action mentioned hereinbefore may varyaccording to the thickness of the protective layer to be obtained. Ifthe duration is too short, the layer is insufiiciently thick; if it istoo long, the layer is too thick but a decrease of adherence is observedand it may have a tendency to scale.

The operations are carried out at temperatures of from 0 to 50 C. andpreferably at room temperature.

I claim:

1. A process of providing a protective coating upon a metallic articlecomposed substantially entirely of magnesium and magnesium base alloy,by galvanic action, which comprises placing such an article to beprotected, as anode in an acid electrolyte having a pH value above 2.5,but not substantially above 3.95, and such electrolyte being incapableof chemically attacking metallic magnesium, and said electrolyteconsisting essentially of water containing a soluble manganese compoundof chromic acid, said electrolyte also containing a small amount of atleast one substance containing the anion of a mineral acid which mineralacid would in higher concentration be capable of chemically attackingmagnesium, such acid being selected from the group consisting ofsulphuric acid, nitric acid and phosphoric acid, the amount of suchmineral acid ion being not more than 0.1% of sulphate ions, not morethan 0.5% of nitrate ions, not more than 0.2% of phosphate ions, andplacing a cathode in said electrolyte, but out of physical contact withsaid anode, such cathode being an electrically conductive material morenoble than,

magnesium, electrically connecting said anode and said cathode outsidesaid bath, the electrode "i6 apotential 10f: rthe: v"magnesium;:electrode :inasaid iielectrolyte relatively; to: .the hydrogen; electrode.:.:-b'eing.=: not over .1.Lavolts,rtcontinuing' the=fiowr0fJsaidocurrentx'until a. protective .film of substan- .cof SOrt radical,the potential atsaidmagnesium eano'dew in :saidielectrolyte: relatively.to theJhy'dro- -.'gen-;electrode :being:1ower..-than .1 .1 1 volts, and:the pI-Irof the electrolyte. being higher than 2.5;1but not above.3.95,..and rsaid .electrolyte being isfreeotmotherusubstances.whichwvould affect the pH and the electrodepotential of said magnesium anode.

-:;.3.-.- A;-.process-, as T covered in, 1 claim ,1, ,in which thecathode is carbon.

4. A process of providing a protective coating upon a metallic articlecomposed substantially entirely of magnesium and magnesium base alloy,by galvanic action, which comprises placing such an article to beprotected, as anode in an acid electrolyte having a pH value above 2.5,but not substantially above 3.95, and such electrolyte being incapableof chemically attacking metallic magnesium, and said electrolyteconsisting essentially of water containing a soluble manganese compoundof chromic acid, said electrolyte also containing a small amount but notover 0.1% of a substance containing S04 ions, and placing a cathode insaid electrolyte, but out of physical contact with said anode, suchcathode being an electrically conductive material more noble thanmagnesium, electrically connecting said anode and said cathode outsidesaid bath, the electrode potential of the magnesium electrode in saidelectrolyte relatively to the hydrogen electrode being not over 1.1volts, continuing the flow of said current until a protective film ofsubstantial thickness has been formed on said anode.

5. A process of providing a protective coating upon a metallic articlecomposed substantially entirely of magnesium and magnesium base alloy,by galvanic action, which comprises placing such an article to beprotected, as anode in an acid electrolyte having a pH value above 2.5,but not substantially above 3.95, and such electrolyte being incapableof chemically attacking metallic magnesium, and said electrolyteconsisting essentially of water containing a soluble manganese compoundof chromic acid, said electrolyte also containing a small amount, butnot over 0.2% of a compound containing P04. ions, and placing a cathodein said electrolyte, but out of physical contact with said anode, suchcathode being an electrically conductive material more noble thanmagnesium, electrically connecting said anode and said cathode outsidesaid bath, the electrode potential of the magnesium electrode in saidelectrolyte relatively to the hydrogen electrode being not over 1.1bolts, continuing the flow of said current until a protective film ofsubstantial thickness has been formed on said anode.

6. A process of providing a protective coating upon a metallic articlecomposed substantially entirely 'of magnesium and magnesium base alloy,

substantially above 3.95, and such electrolyte being incapable ofchemically attacking metallic magnesium, and said electrolyte consistingessentially of water containing a soluble manganese compoundof chromicacid, said electrolyte also containing a small amount but not over 0.5%of a compound containing N03 ions, and placing a .cathode in saidelectrolyte, .but out of physical contact with said anode, such cathodebeing an electrically conductive material more noble than magnesium,electrically connecting said anode' and said cathode outside said bath,the electrode potential of the magnesium electrode in said electrolyterelatively to the hydrogen electrode being not over 1.1 volts,continuing the flow of said current until a protective film ofsubstantial thickness has been formed on said anode.

HERBERT MANFRED FREUD.

REFERENCES CITED The following references are of record inthe file ofthis patent:

' UNITED STATES PATENTS Number Name Date 1,838,273 McBride Dec. 29, 19312,052,962 Booe Sept. 1,1936 2,206,028 Buzzard July 2, 1940 2,224,528Sutton etal Dec. 10, 1940 2,356,575 Frasch Aug. 22, 1944 2,463,483Frasch Mar. 1, 1949 FOREIGN PATENTS Number Country Date 7 515,648 GreatBritain Dec. 11, 1939 702,266 France Apr. 3,1931 471,053 Germany Jan.17, 1929,

OTHER REFERENCES AlienProperty Custodian publication 297,191

20 v 87 through 91.

Transactions of The Electrochemical Society," vol. 90 (1946), pages 503through 507.

1. A PROCESS OF PROVIDING A PROTECTIVE COATING UPON A METALLIC ARTICLECOMPOSED SUBSTANTIALLY ENTIRELY OF MAGNESIUM AND MAGNESIUM BASE ALLOY BYGALVANIC ACTION WHICH COMPRISES PLACING SUCH AN ARTICLE TO BE PROTECTED,AS ANODE IN AN ACID ELECTROLYTE HAVING A PH VALUE ABOVE 2.5, BUT NOTSUBSTANTIALLY ABOVE 3.95, AND SUCH ELECTROLYTE BEING INCAPABLE OFCHEMICALLY ATTACKING METALLIC MAGNESIUM, AND SAID ELECTROLYTE CONSISTINGESSENTIALLY OF WATER CONTAINING A SOLUBLE MANGANESE COMPOUND OF CHROMICACID, SAID ELECTROLYTE ALSO CONTAINING A SMALL AMOUNT OF AT LEAST ONESUBSTANCE CONTAINING THE ANION OF A MINERAL ACID WHICH MINERAL ACIDWOULD IN HIGHER CONCENTRATION BE CAPABLE OF CHEMICALLY ATTACKINGMAGNESIUM, SUCH ACID BEING SELECTED FROM THE GROUP CONSISTING OFSULPHURIC ACID, NITRIC ACID AND PHOSPHORIC ACID, THE AMOUNT OF SUCHMINERAL ACID ION BEING NOT MORE THAN 0.1% OF SULPHATE IONS, NOT MORETHAN 0.5% OF NITRATE IONS, NOT MORE THAN 0.2% OF PHOSPHATE IONS, ANDPLACING A CATHODE IN SAID ELECTROLYTE, BUT OUT OF PHYSICAL CONTACT WITHSAID ANODE, SUCH CATHODE BEING AN ELECTRICALLY CONDUCTIVE MATERIAL MORENOBLE THAN MAGNESIUM, ELECTRICALLY CONNECTING SAID ANODE AND SAIDCATHODE OUTSIDE SAID BATH, THE ELECTRODE POTENTIAL OF THE MAGNESIUMELECTRODE IN SAID ELECTROLYTE RELATIVELY TO THE HYDROGEN ELECTRODE BEINGNOT OVER 1.1 VOLTS, CONTINUING THE FLOW OF SAID CURRENT UNTIL APROTECTIVE FILM OF SUBSTANTIAL THICKNESS HAS BEEN FORMED ON SAID ANODE.